Fundamentals of Computer Networks ECE 478/578 Lecture #9: Collision Resolution and Collision Free Protocols Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona

Collision Resolution Protocols Assume collision between only two packets All new arrivals defer from transmission Node transmits at the next slot with prob ½ Let X denote the # of slots req to resolve collision 2 Throughput = 2/3

Tree Splitting Algorithm 3 Set S R LRL LL LRRL LR LRR LRRR SlotXmit SetWaiting SetFeedback 1S-e 2LRe 3LLLR, R1 4LRRe 5LRLLRR, R0 6LRRRe 7LRRLLRRR,R1 8LRRRR1 9R-0 L idle collision success collision idle collision success Resolution of collision of three stations New arrivals accumulate during contention period Throughput approx 0.43 p/s (need 43 slots for 10 packets)

Improvements 4 Set S R LRL LL LRRL LR LRR LRRR L idle collision success collision idle collision success Guaranteed to have a collision here Two collisions in a row, R must have a small number of packets Incorporate to new arrivals

First Come First Serve Splitting 5 k k+1 current time k+2 k+3 allocationwaiting allocation waiting allocation Split right away two collisions current time

Multi-Access Reservations Idea: Use reservation (contention) slots to make reservations for later transmissions Reservations much smaller size than data Any method can be used to contend for reservation slots (or not contend) 6

Contention lasts N slots (equal to # of stations) Station “i” transmits one bit in slot i if it has a frame to transmit After all the N slots are completed, every station knows who has frames to transmit, start data transmission in that order Example: Assume there are 8 stations Lower numbered stations have to wait for more time Efficiency d/(d + N), d = length of data frame Example: Satellite Reservation System 7

Inefficiency of Reservation System What if the number of stations N is large? Nodes have to wait for a long time to send data Reservation period grows very large 8

Binary Countdown Protocol Each station is assigned a binary address of bits Stations broadcast their address one bit at the time, if they have a frame to transmit Take the OR of all bits wanting to transmit (nodes know the outcome of OR based on electric property of medium) If node with higher address transmits, back off 9 Drop off after slot 0 Drop off after slot 2 Winner Efficiency:

(Un)Fairness of Binary Countdown Stations with highest addresses always win Good for implementing a priority scheme To make the protocol fair, addresses of hosts need to be dynamic 10

Implementing Fairness After node A transmits, all hosts with address less than A increase their address by one Node’s A address is set to zero All nodes before A get a higher priority Round robin type of protocol 11